Control diet: 0.1% of calories were from mixed trans fats, both naturally and industrially produced.

Each diet lasted for 24 days and was completely controlled. All the diets had the same macronutrient composition: 34% of calories from fat, 50% from carbs and 17% from protein.

The additional calories from trans fats replaced calories from stearic acid, a common fatty acid that has neutral effects on both LDL-cholesterol and HDL-cholesterol.

The study had a crossover design, meaning that each participant was in each of the groups on different occasions. At the start and end of each of the diets, the researchers measured blood lipids, inflammatory markers and body weight.

A total of 116 participants started the study, and 91% completed at least one of the four study periods.

When levels of cholesterol become too high, it is often because of increased concentration of low-density lipoproteins (LDL).

Cholesterol carried around in LDL (LDL-cholesterol) is considered especially unhealthy when its levels are abnormally high.

In the present study, LDL-cholesterol increased during both the iTF and the VA diets, compared to the control. The rise in LDL-cholesterol, however, was significantly higher during the VA diet, as shown in the chart below.

These results are supported by a previous study showing that a diet high in vaccenic acid increased LDL-cholesterol (8).

Apolipoprotein B

Apolipoprotein B (apoB) is a class of proteins found on the surface of low-density lipoproteins (LDL). High levels are associated with an increased risk of heart disease.

In the current study, both the VA and the iTF diets increased levels of apoB, compared to the control. However, apoB increased more during the VA diet, as shown in the chart below.

Bottom Line: Both types of trans fats raised levels of LDL-cholesterol and apoB, but VA caused larger increases to both.

Finding 5: Natural Trans Fat Decreased Fibrinogen

Fibrinogen is a type of protein that helps the blood clot. As a result, high levels of fibrinogen are associated with an increased risk of heart disease (12, 13).

In the present study, the VA diet lowered fibrinogen, compared to iTF and the control diet.

High levels of fibrinogen have been associated with systemic inflammation. In this study, however, levels of inflammatory markers remained unchanged across diets.

Bottom Line: Levels of fibrinogen decreased when the participants were on a diet high in vaccenic acid. Industrially produced trans fats had no effects.

Limitations

This study did not appear to have any serious limitations, but a few issues should be mentioned.

First, the VA and iTF diets were supposed to provide the same amount of calories from trans fat. However, these two diets actually differed by 0.67%.

The VA diet provided 3.86% of calories from vaccenic acid, whereas the iTF diet provided 3.26% of calories from industrially produced trans fats. These differences might have affected the results.

Second, the study tested much higher amounts of trans fat than are normally consumed in the USA.

Certain subgroups of people may reach similar intake levels for industrially produced trans fats. The amount of vaccenic acid used, however, exceeded normal dietary intakes by far.

Finally, the dietary context may play a role. In this study, the researchers used synthetic vaccenic acid that was not consumed with dairy fat or meat.

Previous studies suggest that vaccenic acid does not affect the risk of heart disease when eaten in normal amounts with milk or meat (14, 15, 16).

Bottom Line: The study did not have any important limitations.
However, the VA diet provided a greater proportion of calories from trans fat, compared to the iTF diet.

Summary and Real-Life Application

In short, this study shows that both industrially produced trans fats and vaccenic acid, the most common natural trans fat, adversely affect the blood lipid profile, when consumed in equal amounts.

However, this does not mean that natural foods containing natural trans fats need to be avoided. Studies indicate that when eaten in normal amounts with milk or meat, vaccenic acid does not impair heart health (14, 15, 16).